This invention relates to a monitoring system for an electronic instrument and more particularly to a monitoring system for an electronic instrument including within itself a microprocessor (MPU) and a watchdog (WD) circuit each adapted to monitor the operations of the other.
Electronic instruments of which safety and high reliability are required must be able to detect a malfunction or a runaway condition of their main control circuit and stop the operation under a safe condition. For this reason, a so-called watchdog circuit is sometimes incorporated for monitoring the operation of the main control circuit from outside such that an abnormal condition can be detected and an alarm circuit is provided to output a warning signal or stop the operation of the instrument under a safe condition in response to an output from the watchdog circuit.
Although watchdog circuits of different types have been proposed, let us consider as an example for the purpose of explanation an instrument having a microprocessor in its main control circuit. In a typical conventional monitoring system, the microprocessor outputs a periodic check pulse to a watchdog circuit to show its condition of operation and the watchdog circuit receives and analyzes this pulse to conclude that the microprocessor is operating normally. If there is an occurrence of abnormality in the microprocessor, it ceases to operate normally and the check pulse transmitted therefrom to the watchdog circuit becomes irregular or may even stop completely. This is how the watchdog circuit comes to detect an abnormal condition in the microprocessor.
Such a watchdog circuit, however, is no more than a unidirectional monitoring device which makes use of a unidirectional signal from the microprocessor to itself. Since such a unidirectional monitoring system becomes unable to detect an abnormal condition in the microprocessor if its own watchdog circuit ceases to operate normally, bi-directional monitoring systems have come to be considered in order to improve monitoring capability. In a bi-directional monitoring system, not only does the watchdog circuit monitor the microprocessor but also the microprocessor monitors the operation of the watchdog circuit, both outputting a check pulse signal to the other to check whether the other is operating normally. Thus, if an abnormal condition occurs either in the microprocessor or in the watchdog circuit, it can be detected by the other in such a system. Prior art bi-directional monitoring systems are adapted to check merely whether or not check pulses are being received, however, and are not capable of high-precision monitoring.